A capacitor is a passive electrical component that can store energy in an electric field between a pair of conductors. The process of storing energy in the capacitor is known as “charging,” and involves electric charges of equal magnitude, but opposite polarity, building up on each conductor. A capacitor's ability to store charge is measured by its capacitance in units of farads.
Capacitors are often used in electric and electronic circuits as energy-storage devices. They can also be used to differentiate between high-frequency and low-frequency signals. Practical capacitors have series resistance, internal leakage of charge, series inductance and other non-ideal properties not found in a theoretical, ideal, capacitor.
Some capacitors include two conductive electrodes, or plates, separated by a dielectric, which prevents charge from moving directly between the plates. Charge may, however, move from one plate to the other through an external circuit, such as a battery connected between terminals of the plates. When any external connection is removed, the charge on the plates persists. The separated charges attract each other, and an electric field is present between the plates.
An example capacitor may include two wide, flat, parallel plates separated by a thin dielectric layer. Assuming the area of the plates, A, is much greater than their separation, d, the instantaneous electric field between the plates, E(t), is generally the same at any location away from the plate edges. If the instantaneous charge on a plate, −q(t), is spread evenly, thenE(t)=q(t)/εA   (1)where ε is the permittivity of the dielectric. The voltage, v(t), between the plates is given byv(t)=∫odE(t)dz=q(t)d/εA   (2)where z is a position between the plates.
A capacitor's ability to store charge is measured by its capacitance, C, which is the ratio of the amount of charge stored on each plate, q, to the voltage, v:C=q/v   (3)or, substituting (2) into (3):C=εA/d.   (4)
In SI units, a capacitor has a capacitance of one farad when one coulomb of charge stored on each plate causes a voltage difference of one volt between its plates. Capacitance, however, is usually expressed in microfarads (μF), nanofarads (nF) or picofarads (pF). In general, capacitance is greater in devices with large plate areas, separated by small distances. When a dielectric is present between two charged plates, its molecules become polarized and reduce the internal electric field and hence the voltage. This allows the capacitor to store more charge for a given voltage: a dielectric increases the capacitance of a capacitor by an amount proportional to the dielectric constant of the material.